Due to the considerable amount of heat generated during operation of the dynamoelectric machine, efficient cooling of the rotor and stator components is required. The design of an efficient cooling system is particularly acute in large scale hydroelectric units having a capacity of 20 MVA or higher. Past attempts at providing efficient cooling of such machines met with varying degrees of success. These attempts involved the use of cooling gases such as air in conjunction with water cooling or involved a two component gas ventilation system using separate cooling circuits for the rotor and stator components respectively.
In U.S. Pat. No. 3,588,557 of Kilgore et al issued June 28, 1971, the ventilation system included a shroud that extends onto the rotor spider from the stator frame to define an air inlet. Air is drawn down between the rotor poles and the air gap between the rotor and stator. The heated air is then directed through a heat exchanger for cooling and recirculation. This system does not provide transverse cooling through the rotor windings. It also requires a separate cooling system for the stator.
In U.S. Pat. No. 4,182,966 of Mishra et al issued Jan. 8, 1980, a system is disclosed for cooling the stator components only. Air is drawn in to a passage between the stator shroud and rotor spider. An air stream passes over the end windings of the stator and through coolers, after which it travels in an inward direction through the stator windings. This stream is passed through longitudinal ducts in the stator which are sealed off from the rotor and the gap, to provide cooling of the stator independent of contamination from heated air discharging across the gap. While this provides an improved stator cooling, a separate cooling systems is needed for the rotor.
In U.S. Pat. No. 4,233,533 of Lown issued Nov. 11, 1980, a system is disclosed wherein air enters through the rotor spider structure and is directed transversely through ventilation ducts in the rim of the rotor to remove heat from the rotor field windings and poles. This air continues through the gap between the rotor and stator ventilation ducts in order to remove heat from these components as well. Since the cooling air collects heat from the rotor before it reaches the stator ducts, it is difficult to provide efficient cooling of the stator in large dynamoelectric machines. Large volumes of air must pass through the system. This is done by means of directional vanes that provide a tangential velocity to the air and reduce windage loss.
In Canadian Patent No. 1,238,988 issued July 5, 1988 filed in the name of the present applicant, a further attempt is made to provide efficient cooling with reduced windage loss. Turning vanes direct air into the rotor in the direction of rotation and a second set of vanes direct the air from the rotor in reverse direction for recirculation. A second stream is provided axially along the gap and through the stator windings. While this provides more efficient reduction of windage loss than the prior art, the system calls for separate cooling streams for the stator and the rotor.